In electric power-steering apparatuses, an electric motor for producing a supplementary force is fixed to a mechanical steering apparatus, and the rotational torque provided by the motor is controlled using a control device, whereby the steering torque that must be applied by the driver is reduced. In conventional electric power-steering apparatuses, a steering-torque detecting part is provided to a steering shaft linked to the steering wheel. The steering-torque detecting part supplies detection signals to the control device in order to cause the motor to produce an appropriate supplementary steering torque.
Torsion-bar torque sensor systems that make use of the torsion of a torsion bar are the conventional steering-torque detecting parts that have been primarily used. Magnetostrictive torque sensor systems have also been proposed in recent years.
In magnetostrictive torque sensor systems, a magnetostrictive film formed of, e.g., a Ni—Fe plating is provided to two locations on the steering shaft. The magnetostrictive films in these two locations are both formed annularly in the circumferential direction on the surface of the steering shaft and are positioned vertically relative to one another in the axial direction. The magnetostrictive films in these two locations are also formed so as to have the necessary width in the axial direction and are made so as to be magnetically anisotropic in mutually opposing directions. When the driver applies a steering torque to the steering shaft, changes in the magnetostrictive characteristics that occur based on the magnetic anisotropy of the magnetostrictive films in these two locations are detected by a coil provided around the magnetostrictive films.
Such magnetostrictive torque sensor systems are disclosed in, e.g., JP-A 2001-133337 and JP-A 2002-168706.
The magnetostrictive torque sensor systems described in JP-A 2001-133337 and JP-A 2002-168706 have a magnetizing coil and detection coil that are provided respectively to the two annular magnetostrictive films formed on the surface of the steering shaft. There are also magnetostrictive torque sensor systems that detect torque according to changes in the inductance of the detection coil, using only a detection coil without a magnetizing coil (see, e.g., JP-A 2002-71476 and JP-A 2005-321316).
The detection circuit of the magnetostrictive torque sensor system described in JP-A 2002-71476 has a coil around the magnetostrictive films formed on the surface of the steering shaft and also has a resistance element and a switching element that are serially connected to the coil. A power source that applies the necessary voltage is provided to the coil. A bottom-hold circuit for maintaining the minimum value of the output signal is also connected to the connecting part between the resistance element and the coil.
JP-A 2005-321316 discloses a magnetostrictive torque sensor system that improves on the magnetostrictive torque sensor system described in JP-A 2002-71476. This magnetostrictive torque sensor system also has a coil positioned around the magnetostrictive films, as well as a resistance element and a switching element that are serially connected to the coil.
In the conventional magnetostrictive torque sensor systems disclosed in, e.g., JP-A 2005-321316, the frequency of the on/off operation of the switching element is, e.g., about 30 kHz. The frequency of the change in the voltage signal retrieved from the terminal of the coil that responds to changes in the magnetic characteristics of the magnetostrictive films is also about 30 kHz. As a result, the detection period of the peak-hold circuit used in the detection circuit can be calculated as the inverse of 30 kHz. When the detection period of the peak-hold circuit is in a frequency range of 30 kHz, then in terms of the frequency characteristics, the gain usually decays and phase lag increases. An increase in the phase lag in the output signal of the magnetostrictive torque sensor system results in reduced stability with which the electric power-steering apparatus is controlled, and greater loss in the uniformity of the supplementary force for reducing the steering torque that must be applied by the driver. Problems result in that the entire steering torque loses consistency, and the driver increasingly loses proper steering response.
A magnetostrictive torque sensor system has therefore been needed for stabilizing the steering torque applied to the steering shaft and performing detection, without affecting the frequency characteristics of the peak-hold circuit used in the detection circuit of conventional magnetostrictive torque sensor systems and without giving rise to gain decay or phase lag in the torque-detection signal. A demand has also arisen for an electric power-steering apparatus that can provide a good steering response using a magnetostrictive torque sensor system.